Power control techniques are commonly used in wireless communication systems for reducing interference levels, prolonging battery life, and/or reducing dynamic range requirements of the base station receiver. Conventional power control systems generally cutback the transmission power of the mobile station when it is in close proximity to the serving base station. The amount of cutback applied is usually inversely proportionate to the distance between the base station and the mobile station. In other words, more cutback of the transmission power would be applied as the mobile station gets closer to the base station. This general method of power control is quite common and is generally known in the art. Moreover, since current cell phones have many similar functions to that of computer devices, a mobile station will be herein used to refer to any device that requires a power control system, which includes, but is not limited to, cell phones, personal digital assistants, and/or computers.
Digital communication systems often employ linear signaling methods to obtain maximum information rate in a limited band. In these methods, information is contained in the phase and amplitude components of the signal. This type of signaling, however, imposes strict linearity requirements on the transmitter power amplifier (“PA”). Despite such requirements, the PAs can only operate linearly over a limited range of signal levels. If the dynamic range of the input signal exceeds the linear operating range of the PA, nonlinear distortion will result, which causes signal splatter and potential loss of information. Thus, systems often employ dynamic range control techniques in order to ensure this type of power amplifier linearity is maintained. A common technique is to attenuate signal peaks, and thereby limit the range of input levels to the PA. This type of approach generally reduces the peak to average ratio (“PAR”) of the signal, and there are several well known benefits of PAR control in amplitude modulated systems. For example, some of those benefits include a maximized average transmit power, a higher PA efficiency, a longer battery life, and a reduced system cost.
In particular, FIG. 1 shows a conventional system employing PAR control and power cutback, which is indicated generally at 100. In this particular system shown, there are two major controls, which are a PAR control 102 that reduces the peak level of the signal and a power control 104 that controls the transmit power of a connected receiver 106, that controls the power of the PA 108. On the side of the PAR control 102, other typical component circuits are found coupled to the PAR control. For example, a data source circuit 110 that provides data to be transmitted is operably coupled to an encoder circuit 112 that protects the data from channel impairments. The encoder circuit 112 is then coupled to a modulator circuit 114 that formats the data for signaling across the channel, and the PAR control 102 is used to control the peak level of the signal from the encoder circuit 112.
In order to control the peak level of the signal, the PAR control circuit 102 uses a parameter tclip to establish a threshold for peak limiting. Whenever the signal envelope exceeds this tclip parameter, the PAR control circuit 102 applies a controlled amount of attenuation or clipping centered about the peak of the envelope. In this case, larger peaks are attenuated more than smaller peaks. As a result, a reduced and relatively constant PAR at the output of the PAR control circuit 102 is generated, enabling the PA 108 to operate within its linear range. Both outputs from the PAR control circuit 102 and power control circuit 104 are forwarded to a variable gain amplifier 116, which controls the transmit power, that outputs a baseband signal to a digital-to-analog (“D/A”) converter circuit 118. A mixer 120 that is coupled to the D/A converter circuit 118 and a local oscillator 122 then translates the signal to a radio frequency (“RF”) signal to the PA 108, which is coupled to an antenna 124 for forwarding the RF signal onto the channel.
Turning now to FIG. 2, the function of the power control circuit 104 in a conventional transceiver design is shown and indicated generally at 200. An input of the system's current operating cutback level 202, which is signaled to the transmitter through the receiver, is received from the receiver 106 (shown in FIG. 1). As shown, the current operating cutback level 202 represented in dB is mapped into a linear gain control circuit 204, which outputs a gain control signal 206 to the variable gain amplifier 116 (shown in FIG. 1). This gain control signal 206 is reduced in proportion to the amount of cutback applied by the system. Although, as shown, both the PAR control circuit 102 and the power control circuit 104 ultimately controls the power of the PA 108, each of the circuits has no idea what the other circuit is doing to the power adjustment of the system. Since the PAR control 102 circuit and the power control circuit 104 operate independently of one another, the PAR control circuit is set up with a fixed tclip threshold and fixed gain stage optimized for the desired PAR and fixed peak signal required for the input dynamic range of the next stage. The tclip and gain settings are fixed regardless of the cutback being applied. As a result, a separate power control circuit 104 is implemented to apply all of the attenuation needed for the system cutback.
Moreover, the PAR control circuit 102 introduces additional problems itself. For example, the PAR control circuit 102 introduces nonlinear distortion into the system. The distortion, in turn, increases the splatter of the signal and introduces an irreducible bit error rate (“BER”) floor. As a result, the overall quality of the transmitted signal is limited. Although the amount of distortion can typically be controlled at acceptable levels, the degradations can negatively affect system performance in several areas. Specifically, splatter can interfere with other users in close proximity on neighboring channels, and irreducible BER floor can degrade audio quality in voice systems and/or data throughput rates in data systems. Moreover, reduction in signal quality can also degrade mobility performance of the system. As a result of the PAR control circuit 102 being disconnected from the power control circuit 104, the tradeoff between signal quality and output power whenever the mobile station is not in a power limited environment is not being optimized.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.